The present invention relates generally to the field of optical amplifiers and lasers. More particularly, the present invention relates to methods and systems related to optically excited rare-earth doped optical fiber gain medium. Merely by way of example, the methods and systems have been applied to reducing out-of-band gain and amplified spontaneous emission in optical fibers. But it would be recognized that the invention has a much broader range of applicability.
Conventional laser-based material processing has generally used high peak power pulsed lasers, for example, Q-switched Nd:YAG lasers operating at 1064 nm, for marking, engraving, micro-machining, and cutting applications. More recently, laser systems based on fiber gain media have been developed. In some of these fiber-based laser systems, fiber amplifiers are utilized.
Some optical amplifiers and lasers utilizing a fiber gain medium are optically pumped, often by using semiconductor lasers pumps. The fiber gain medium is typically made of silica glass doped with rare-earth elements. The choice of the rare-earth elements and the composition of the fiber gain medium depends on the particular application. One such rare-earth element is ytterbium, which is used for optical amplifiers and lasers emitting in the 1020 nm-1100 nm range. Another rare-earth element used in some fiber gain medium is erbium, which is used for optical amplifiers and lasers emitting in the 1530 nm-1560 nm range.
The wavelength of the optical pump source used for ytterbium-doped fiber amplifiers and lasers is typically in the wavelength range of 910 nm to 980 nm. The wavelength of the optical pump source used for erbium-doped fiber amplifiers and lasers is typically in a wavelength range centered at about 980 nm or about 1480 nm. When ytterbium-doped or erbium-doped fiber amplifiers are pumped at the above mentioned wavelengths, they generally have significant gain and amplified spontaneous emission (ASE) outside of the wavelength range of interest, i.e., the lasing or amplification wavelength. For example, when an ytterbium-doped fiber gain medium is pumped at a wavelength of about 915 nm, it generates high gain and ASE at about 976 nm; when it is pumped at a wavelength of around 976 nm, it generates high gain and ASE at about 1030 nm. In the case of erbium-doped fiber, pumping at wavelengths of 980 nm or 1480 nm generates high gain and ASE at around 1530 nm.
As a result of the out-of-band gain, i.e., the gain present outside the wavelength range of interest, it is possible for the amplifiers or the lasers to produce ASE or start lasing at these out-of-band wavelengths. Such ASE or lasing will limit the amount of gain available at the wavelength of interest. In some amplifier applications, large out-of-band ASE will limit the available gain and the ASE power may be larger than the signal power at the wavelength of interest.
Thus, there is a need in the art for fiber-based amplifiers and lasers with reduced out-of-band ASE and gain.